The present invention applies to the field of coating substrates using processes in which a high level of coating uniformity is required. Such processes may use physical vapor deposition (PVD) or sputtering to apply the coating.
Typical coating processes that achieve a high level of uniformity generally use an arrangement similar to the arrangement shown in FIG. 1. A process that uses this arrangement is typically called a xe2x80x9clong throwxe2x80x9d process, because there is often a considerable distance between the source of coating material and the substrates. In FIG. 1, the source 1 is shown as a cylindrical can representative of a vapor source for a PVD process in which the can contains the material being evaporated. The can is substantially a point source of material. In a long throw sputtering process, the source is typically a sputtering target, which is usually larger than the evaporative source. As disclosed by FIG. 1, the typical arrangement includes substrates 2 carried by a platen 5. Several platens may be mounted on a rack 3.
To achieve a uniform coating on the substrates, two distinct motions are typically applied to the substrates 2. The first motion is provided by rotation of the rack 3 about the axis 4. The second motion is provided by rotation of the platen 5, which holds the substrates, about its axis 6. The compound motion produced by the combination of the first and second motions is called xe2x80x9cplanetary rotationxe2x80x9d.
In processes employing planetary rotation, the rack and platen have different rates of rotation. Radial reference lines 7 and 8 have been drawn on the rack 3 and platen 5, respectively and a reference line 9 connects the two axes of rotation. As the rack 3 and platen 5 rotate, the angles that the projections of the lines 7, 8 and 9 make will change. At a first instant of time the lines 7 and 8 make certain angles with the line 9. If the motions of rack and platen are generated by mechanical means such as gears or chains connected to the same source of motion, then at some later second instant of time lines 7 and 8 will make the same angles with the line 9. The time interval between the second and first instants of time (during which both rack and platen will complete a whole number of rotations) may be referred to as the period. The first and second motions are selected such that a large number of revolutions of both rack and platen occurs during the period. This selection causes all substrates mounted at the same distance from the axis 6 of the second rotation to experience almost exactly the same path within the chamber during one period. Therefore, the coating applied to all substrates equidistant from the center will be the same.
In the process disclosed in FIG. 1, the uniformity of the coating that is applied to a point on a substrate varies with the distance from that point to the axis of the second motion. In order to achieve a given level of uniformity, the region where the substrates may be placed is limited to the space between circles 10 and 11. As the requirement for uniformity becomes more stringent, or as the deposition becomes less uniform, the radial distance between the circles decreases, limiting the number and size of substrates that may be coated in a single process.
For processes requiring uniform deposition over a large area, the distance between the source 1 and the substrate 2 is generally considerably greater than the radial distance between circles 10 and 11. In addition, masks, such a sector masks, which may move in a third motion about the axis of the second rotation 6, or fixed xe2x80x9cwallxe2x80x9d masks, may be used to improve uniformity. The large distance between the substrate 2 and the required masks reduces the deposition rate of the process, resulting in a long and expensive process to produce a limited number of coated substrates.
Another process currently employed is commonly referred to as a xe2x80x9cshort throwxe2x80x9d sputtering processes. In a short-throw process, the distance between the source of material (sputtering target) and the substrates is usually only a few inches. These short throw processes include xe2x80x9cbatch processesxe2x80x9d in which the substrates are transported past a source of coating material by a rotating drum and xe2x80x9cin-linexe2x80x9d processes in which a transporting mechanism carries the substrates past the source in a substantially straight path. Such processes are widely used in industry to apply coatings to substrates. For example, U.S. Pat. No. 5,714,009 to Bartolomei, commonly assigned with the present application, discloses such a process. The Bartolomei patent, incorporated by reference herein, describes arrangements for producing coatings by microwave-assisted sputtering. In the disclosed process, both rotating drums and linear transport mechanisms are used to transport substrates past sputtering targets and microwave energized plasma generators in a reactive sputtering process. FIG. 2 depicts one of the possible arrangements.
Referring to FIG. 2, a sputtering chamber 21 contains a rotatable drum 22 which carries substrates 23 in a first motion parallel to the direction of the arrow 24 past an elongated sputtering target 25 and past an elongated microwave-energized plasma generator 26. The substrates 23 are arranged in rows that are parallel to the substrate motion and columns perpendicular to that motion. The target 25 and plasma generator 26 are typically mounted on the chamber wall, and are visible in FIG. 2 because a portion of the wall has been cut away. Other sputtering targets and plasma generators, not shown, may also be mounted on the chamber wall. Usually additional targets and plasma generators will have the same vertical dimensions and will be mounted in the same vertical position as the targets and generators shown in FIG. 2.
During the sputtering process, material will be sputtered from the sputtering target 25 on to the substrates 23 where it will react with a reacting gas in the chamber to produce the desired coating. It is almost always necessary to assure that all of the substrates receive a coating that has nearly the same properties. In particular, the amount of deposited material per unit area on each substrate must generally be the same within a prescribed limit.
The amount of material deposited on a given substrate depends on the location of the substrate in the direction of the longer length of the target. The arrow 27 indicates this direction, referred to throughout the application as the xe2x80x9cz directionxe2x80x9d. The deposition of material is highest at the center of the sputtering target and decreases to zero at extreme distances from the center. In FIG. 2, the lines 28 and 29 at the ends of arrow 27, bound the region within which uniformity of deposition remains within tolerance. It is typically necessary to restrict the size of the region in the z direction so that the difference between the deposition on the center substrates and on the end substrates lies within the acceptable tolerance. Thus, the number of substrates in each column is the number that may be mounted between these limits. This number will be reduced in processes in which a tighter tolerance is imposed.
FIG. 3a is a graph illustrating the correlation between the amount of deposited material and the position of the substrate along the vertical column (i.e. position in the z direction). Curve 30 in FIG. 3a applies to the batch process of FIG. 2 and shows the amount of deposited material per unit area at substrate locations along the z direction. The target generating the curve disclosed in FIG. 3a is assumed to be xe2x80x9cidealxe2x80x9d, that is, it has a uniform rate of sputtering at all locations.
The deposition of material on the substrates is highest at point 31, which lies opposite the center of the target. At locations 32 that lie opposite the ends of the target the deposition is reduced to approximately half of the center value. Arrows 33 are provided to indicate the tolerance for the process. The limits of the area within which substrates may be placed and still meet the tolerance are reached when the difference between the maximum (center) value and the value at the limit equals the tolerance. Lines 34 are provided to show the limits. The tolerance is always considerably less than 50%, and the limits must always be displaced inward from the ends of the elongated target resulting in a region of deposition less than the target length. It should be noted that the rate of sputtering from a real target is not perfectly uniform, therefore, the limits must be moved inward farther than shown in FIG. 3a when considering a real target. A figure similar to FIG. 3a would also apply to the process disclosed in FIG. 1.
The production rate of a coating process is proportional to the number rows of substrates being coated at one time. The number of rows is limited by the target size. Therefore, high production rates require large targets. Large targets are expensive, difficult to maintain, subject to uniformity variations along their length, and require large and expensive power supplies. Furthermore, large targets are more vulnerable to arcing, than small targets. Arcing interferes with the stability of the coating process and degrades the quality of the deposited film.
The location of the substrates in the z direction also affects the extent to which the deposited material combines with the reactant gas. Similar to the requirements for uniformity of deposition discussed above, uniformity of reaction requires that the length of the plasma generator be greater than the width of the region 27 containing the substrates. The relatively long generators required to produce uniform reactions are expensive, difficult to maintain, require costly microwave supplies, and are subject to non-uniformity of plasma generation.
Since uniformity of reaction depends in part on local reactant gas concentration, the sputtering process shown in FIG. 2 requires a system for controlling the flow of reaction gas. The system employs vents which admit the gas at prescribed locations at carefully controlled rates. Flow is regulated by flow controllers which may be actuated by computer generated inputs. The flow control system is often complex and expensive.
FIG. 4 discloses an xe2x80x9cin linexe2x80x9d sputtering process that employs a substrate transport mechanism for moving the substrates in a straight line. The system includes a sputtering chamber 41, which is shown with its wall cut away. The chamber 41 contains the linear transport mechanism 42, such as a belt or web, which carries substrates 43 in a first motion parallel to the direction of the arrow 44 past an elongated sputtering target 45 and an elongated microwave-energized plasma generator 46. The direction of first motion may change direction during a single process as indicated by arrow 44. As shown in FIG. 2, the substrates are arranged in rows that are parallel to the length of the target and perpendicular columns. Both the target and plasma generator are mounted on the chamber wall. Other sputtering targets and plasma generators, not shown, may also be mounted on the chamber wall. These additional targets and generators will normally have the same length and will be mounted in the same position measured along the columns as the target 45 and generator 46 that are shown. Material is sputtered from the sputtering target 45 and then combines with a reacting gas to produce the desired coating. The requirement to achieve a desired level of uniformity limits the area in which substrates may be coated to the area between the lines 47.
The relations between the degree of uniformity and the size, cost and complexity of the batch process of FIG. 2 also apply to the in line process of FIG. 4. In both processes, the substrates move so that the center of each substrate in a given row remains at a fixed distance from a plane that contains the direction of motion and which bisects the process (xy plane). This distance is different for each substrate in a particular column. Conditions such as deposition rate, reactive gas concentration, and plasma density tend to vary as the distance changes. Therefore, it is difficult to achieve a high degree of coating uniformity in either of these process without incurring unacceptable production cost.
Variations based on the concepts illustrated in FIG. 1, 2, and 4 are known to the prior art. U. S. Pat. No. 5,618,388, issued to Seeser et al. and incorporated by reference herein, discloses a variety of coating processes. FIGS. 10 and 11 of the Seeser patent disclose modifications of the process disclosed in FIG. 2 where the top and bottom of the chamber have been moved away from the drum to make the chamber longer and to provide space at both ends of the drum for movement of the drum in a reciprocating motion in the directions indicated by the double-headed arrow 27 of FIG. 2. The reciprocating motion combines with the conventional rotary motion indicated by the arrow 24, causing the substrates to move in a helical path with respect to the chamber and the sputtering targets. The substrates mounted on the top of the drum move in the top portion of the chamber, while the substrates on the bottom of the drum move in the bottom portion of the chamber. The substrates mounted in the center of the drum move in a path that extends into both ends of the chamber. It is apparent that all of the substrates do not travel within the same region of the chamber and, as a result, are not exposed to the same conditions of deposition.
FIG. 3b is a graphical representation of the coating thickness deposited on the substrates using the Seeser process. Curve 30 is a plot of deposition per unit area on a substrate as a function of the distance of the substrate from the center of the target. The distance is measured along the z direction, and the curve 30 applies when no reciprocating motion is present. The location of the center of the target is at the center of the horizontal axis.
When reciprocating motion of the substrates is added, as disclosed in the Seeser patent, the range of z over which deposition occurs is increased. Consequently, the range of z over which deposition is plotted in FIG. 3b has been expanded. The target extends over the range between the lines 32 in FIG. 3a and FIG. 3b. The range of z over which curve 30 extends is somewhat more than a full target length in both directions from the center of the target. As the distance from the center of the target increases above half of the target length, the deposition value given by curve 30 decreases rapidly toward zero.
Considering the process in which the reciprocating motion of the substrates occurs as described above, and where the reciprocating motion has the same constant speed regardless of the direction of rotation of the drum. The reciprocating motion will carry the substrates located at the center of the target through the region between the lines 32 (the xe2x80x9ccenter regionxe2x80x9d). The reciprocating motion has constant velocity, thus, the substrates will receive equal deposition at all locations within the region. The deposition on the substrate may be obtained by computing the average of curve 30 over the center region. This average has been estimated to have the value given by the ordinate of the point 38, whose abscissa is 0, since it represents deposition on a substrate at z=0.
The substrates located at the ends of the target move through one of the regions situated between line pairs 35 or 36, (the xe2x80x9couter regionsxe2x80x9d). The deposition received by each of the end substrates may be calculated by averaging curve 30 over the appropriate outer region. The deposition values for the end substrates are shown by the ordinates of the points 37 and 39 whose abscissas are plus and minus half of the length of the target. The three points 37, 38, and 39 have been connected in FIG. 3b to obtain curve 40 which shows the dependence of the deposition thickness on the location of the substrates relative to the center of the target in the z direction.
The deposition is higher for the substrate at the center of the target than for the substrates at the ends of the target as disclosed in FIG. 3b. The deposition on the center substrate is obtained by averaging over the inner and outer halves of the center region. Over both of these halves the average value is the same relatively high value. The deposition on the outer substrates is obtained by averaging deposition occurring over both halves of the outer regions. The half of each outer region which is nearest the center of the target is the same as one of the halves of the center region, but the half of each outer region which is most remote from the target center has an average deposition value that is much less than the average deposition value of the half nearest the center region. Therefore, the deposition on the center substrate is greater than that on the end substrates and the process does not produce a film of equal thickness on all substrates.
The non-uniform deposition produced by the process disclosed in the Seeser patent occurs even in the case where the target is ideal (i.e, when the rate of emission from the target is constant along its length). However, if the rate of emission varies along the length of the target then the non-uniformity of the Seeser process will increase. In particular, the deposition curve 30 will not be symmetrical about the center of the target. Deposition on substrates equally spaced from the center of the target will no longer be equal.
Accordingly, it is an object of the present invention to obviate the problems of the prior art and provide a novel system and method for depositing more uniform coatings on substrates.
It is another object of the present invention to provide a novel system and method for depositing a layer of material on an array of substrates so that each substrate moves along a common path relative to each of the sources of deposition material.
It is a further object of the present invention to provide a novel system and method for depositing material on a planar array of substrates being moved-in a first rotational motion while concurrently being moved in a second non-rotational motion superimposed on the first motion.
It is still a further object of the present invention to provide a novel system and deposition process in which a planar array of substrates moves in a first motion while concurrently moving in a second motion along a linear path substantially perpendicular to the direction of the first motion.
It is yet another object of the present invention to provide a novel system and method of depositing material on an array of substrates concurrently moving in first and second motions so that each of the substrates move along a common path relative to the source of the deposition material.
It is an additional object of the present invention to provide a novel system and method for short throw deposition in which an array of substrates moves on a first transport mechanism while concurrently being moved in a second motion that does not change the position of the carrier relative to the source of deposition material.
It is still another object of the present invention to provide a novel system and method of depositing a layer of material on an array of elongated substrates being carried on a rotating cylindrical surface, while each substrate is concurrently rotating about its longitudinal axis and being moved in a direction parallel to the axis of rotation of the cylindrical array so that each substrate moves along a common path relative to the sources of deposition material.
It is still a further object of the present invention to provide a novel method and apparatus for depositing a layer of material on an array of substrates the apparatus including a first carrier for moving the substrates in a first motion and a second carrier for concurrently moving the substrates in a second motion so that each of the substrates moves along a common path relative to the sources of deposition material.
It is still another object of the present invention to provide a novel method and apparatus for depositing a layer of material on an array of substrates, the apparatus including a planar rotating first transport mechanism for moving the substrates in a first motion and a second transport mechanism including substrate holders for concurrently moving the substrates in a second motion along a common linear path extending outward from the axis of rotation of the first transport mechanism so that each of the substrates moves along a common path relative to the sources of deposition material.
It is a further object of the present invention to provide a novel method and transport apparatus for moving an array of substrates along a first linear path in a first motion while concurrently moving the substrates in a second motion along a common linear path substantially perpendicular to the path of the first motion.
It is yet another object of the present invention to provide a novel method and transport apparatus for changing the position of an array of substrates carried by a rotating drum relative to the drum.
It is yet another object of the present invention to provide a novel method and transport apparatus for carrying substrate holders along a linear path substantially parallel to the longitudinal axis of the deposition target.
It is still a further object of the present invention to provide a novel substrate holder system and method with removable substrate holders so that the substrate holders may be loaded and unloaded with substrates while other substrates are present in the coating machine.
It is an object of this invention to provide a novel coating system and method using a plurality of relatively smaller sources and plasma generators.
It is a further object of this invention to provide a novel system and method for achieving a high degree of uniformity of reaction in a reactive sputtering process without employing elaborate gas distribution systems.
It is a further object of the invention to provide a novel substrate coating system and method in which the number of substrates being coated is independent of the size of the material sources and other components.
It is a still further object of the invention to provide a novel system and method for both batch and in-line sputtering processes that achieves a high degree of coating uniformity at a low production cost.
It is a still further object of the invention to provide a novel system and method of coating substrates in which the uniformity of the deposited coating is substantially independent of the configuration of the material sources.
It is a still further objective of this invention to provide a novel long throw sputtering system and method with a reduced throw distance and physical masking.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.